Ajax二级联动菜单实现原理及代码

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index.jsp:

 
<%@ page language="java" pageEncoding="UTF-8"%> 
<html> 
<head> 
<title>二级菜单联动演示</title> 
<script type="text/javascript"> 
var req; 
window.onload=function() 
{//页面加载时的函数 
} 
function Change_Select(){//当第一个下拉框的选项发生改变时调用该函数 
var province = document.getElementById('province').value; 
var url = "select?id="+ escape(province); 
if(window.XMLHttpRequest){ 
req = new XMLHttpRequest(); 
}else if(window.ActiveXObject){ 
req = new ActiveXObject("Microsoft.XMLHTTP"); 
} 
if(req){ 
req.open("GET",url,true); 
//指定回调函数为callback 
req.onreadystatechange = callback; 
req.send(null); 
} 
} 
//回调函数 
function callback(){ 
if(req.readyState ==4){ 
if(req.status ==200){ 
parseMessage();//解析XML文档 
}else{ 
alert("不能得到描述信息:" + req.statusText); 
} 
} 
} 
//解析返回xml的方法 
function parseMessage(){ 
var xmlDoc = req.responseXML.documentElement;//获得返回的XML文档 
var xSel = xmlDoc.getElementsByTagName('select'); 
//获得XML文档中的所有<select>标记 
var select_root = document.getElementById('city'); 
//获得网页中的第二个下拉框 
select_root.options.length=0; 
//每次获得新的数据的时候先把每二个下拉框架的长度清0 
for(var i=0;i<xSel.length;i++){ 
var xValue = xSel[i].childNodes[0].firstChild.nodeValue; 
//获得每个<select>标记中的第一个标记的值,也就是<value>标记的值 
var xText = xSel[i].childNodes[1].firstChild.nodeValue; 
//获得每个<select>标记中的第二个标记的值,也就是<text>标记的值 
var option = new Option(xText, xValue); 
//根据每组value和text标记的值创建一个option对象 
try{ 
select_root.add(option);//将option对象添加到第二个下拉框中 
}catch(e){ 
} 
} 
} 
</script> 
</head> 
<body> 
<div align="center"> 
<form name="form1" method="post" action=""> 
<table width="70%" border="0" cellspacing="0" cellpadding="0"> 
<tr> 
<td align="center"> 
二级联动示例 
</td> 
</tr> 
<tr> 
<td> 
<select name="province" id="province" onChange="Change_Select()"> 
<!--第一个下拉菜单--> 
<option value="0"> 
请选择 
</option> 
<option value="1"> 
北京 
</option> 
<option value="2"> 
天津 
</option> 
<option value="3"> 
山东 
</option> 
</select> 
<select name="city" id="city"> 
<!--第二个下拉菜单--> 
<option value="0"> 
请选择 
</option> 
</select> 
</td> 
</tr> 
<tr> 
<td> 
</td> 
<tr> 
</table> 
</form> 
</div> 
</body> 
</html> 

SelectServlet:

复制代码代码如下:

 
package com; 
import java.io.IOException; 
import javax.servlet.ServletException; 
import javax.servlet.http.HttpServlet; 
import javax.servlet.http.HttpServletRequest; 
import javax.servlet.http.HttpServletResponse; 
/*** 
* 
* @author zdw 
* 
*/ 
public class SelectServlet extends HttpServlet 
{ 
private static final long serialVersionUID = 1L; 
public SelectServlet() 
{ 
super(); 
} 
public void destroy() 
{ 
super.destroy(); 
} 
public void doGet(HttpServletRequest request, HttpServletResponse response) 
throws ServletException, IOException 
{ 
// response.setCharacterEncoding("GBK"); 
response.setContentType("text/xml"); 
response.setHeader("Cache-Control", "no-cache"); 
request.setCharacterEncoding("GBK"); 
response.setCharacterEncoding("UTF-8"); 
String targetId = request.getParameter("id").toString(); 
System.out.println(targetId); 
// 获得请求中参数为id的值 
String xml_start = "<selects>"; 
String xml_end = "</selects>"; 
String xml = ""; 
if (targetId.equalsIgnoreCase("0")) 
{ 
xml = "<select><value>0</value><text>请选择</text></select>"; 
} else if (targetId.equalsIgnoreCase("1")) 
{ 
xml = "<select><value>1</value><text>昌平</text></select>"; 
xml += "<select><value>2</value><text>丰台</text></select>"; 
xml += "<select><value>3</value><text>海淀</text></select>"; 
xml += "<select><value>4</value><text>朝阳</text></select>"; 
} else if (targetId.equalsIgnoreCase("2")) 
{ 
xml = "<select><value>1</value><text>塘沽区</text></select>"; 
xml += "<select><value>2</value><text>汉沽区</text></select>"; 
xml += "<select><value>3</value><text>大港区</text></select>"; 
xml += "<select><value>4</value><text>东丽区</text></select>"; 
} else 
{// 如果是3,则返回下面的字符 
xml = "<select><value>1</value><text>济南</text></select>"; 
xml += "<select><value>2</value><text>青岛</text></select>"; 
xml += "<select><value>3</value><text>淄博</text></select>"; 
xml += "<select><value>4</value><text>枣庄</text></select>"; 
} 
String last_xml = xml_start + xml + xml_end; 
response.getWriter().write(last_xml); 
} 
public void doPost(HttpServletRequest request, HttpServletResponse response) 
throws ServletException, IOException 
{ 
doGet(request, response); 
} 
public void init() throws ServletException 
{ 
} 
} 

web.xml:

复制代码代码如下:

 
<?xml version="1.0" encoding="UTF-8"?> 
<web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee 
http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"> 
<servlet> 
<servlet-name>SelectServlet</servlet-name> 
<servlet-class>com.SelectServlet</servlet-class> 
</servlet> 
<servlet-mapping> 
<servlet-name>SelectServlet</servlet-name> 
<url-pattern>/select</url-pattern> 
</servlet-mapping> 
<welcome-file-list> 
<welcome-file>index.jsp</welcome-file> 
</welcome-file-list> 
</web-app> 